A surface light source device such as an EL (Electric Luminescence) backlight, CCFL (Cold Cathode Fluorescent Lamp) backlight, and LED (Light Emitting Diode) backlight used in a liquid crystal display usually has a peak at a certain wavelength.
Accordingly, by arranging a bandpass filter (interference filter), which reflects a certain wavelength of light emitted from a backlight in case of oblique incident light, while allows it to pass through the bandpass filter in case of perpendicular incident light, on a light emitting side, the perpendicular incident light passes through the bandpass filter, while the oblique incident light does not pass therethrough but is reflected thereon. As a result, light can be parallelized (oriented in parallel).
According to the aforesaid bandpass filter, unlike to the light paralleling technique, which utilizes a conventional light shielding plate, non-parallel light rays are not absorbed but reflected, and then returned towards the backlight. Herein, the reflected oblique incident light is returned to the backlight and re-reflected towards the bandpass filter. Then, only a forward component of the re-reflected light passes through the bandpass filter. Accordingly, by a so-called light recycling effect enabling repeating the above actions, the forward (perpendicular) light intensity of the re-reflected light, which passes through the bandpass filter, is enhanced. Thus, it is possible to produce a surface light source device that is capable of emitting parallel light rays at high efficiency.
Herein, the wavelength characteristics of the light interference in the bandpass filter is varied according to the incident angle, that is, the selective wavelength allowed through the bandpass filter is varied according to the incident angle, so that the parallelism of the parallel light rays can be controlled by the transmission center wavelength and the transmission wavelength width (half band width). For example, where a narrow transmission wavelength width (half band width) has been set, passing light is converged only at an extremely narrow point and its proximity on the front side. Thus, a surface light source device with a high parallelism can be produced.
On the other hand, where a broad transmission wavelength width (half band width) has been set, it is possible to make the parallelism approximate to the parallelism produced when a conventionally and commercially available prism sheet for luminance improvement has been used. Herein, given the use of a prism sheet, which utilizes light refraction in an air interface in principle, it cannot be attached to a backlight or liquid crystal cell. However, unlike to the prism sheet, the bandpass filter does not require an air interface and therefore can be integrally attached to a backlight or liquid crystal cell, thereby achieving ease of handling of the entire device. Also, the bandpass filter, which has a smooth surface, can be subjected to hard coat treatment or the like, thereby achieving a scratch proof surface and thus enabling more ease of handling of the entire device. Contrarily to this, the prism sheet, which utilizes refraction on the surface, is hard to be subjected to any scratch prevention treatment such as a hard coat treatment. In view of this, it is a great advantage to utilize a bandpass filter for parallelizing light of a backlight.
As an optical element utilizing such a bandpass filter, for example, those utilizing a cholesteric liquid crystal have been proposed for example in Japanese patent application nos. 2001-60005 and 2000-281382, by which a surface light source device that achieves parallelized light (concentrated light) can be produced.
On the other hand, as a bandpass filter used for parallelizing light of a backlight, it is a matter of course that not only those utilizing the cholesteric liquid crystal but also those made of lamination of vapor-deposited thin films respectively having different refraction factors or those made of lamination of thin films of resin compositions respectively having different refraction factors can be used. Such a bandpass filter is arranged on the emitting side of the backlight so as to achieve an improved efficiency in parallelizing light of a backlight and an improved light utilization efficiency.
A bandpass filter made of lamination of vapor-deposited thin films or thin films of resin compositions is advantageous in the fact that it has excellent heat and chemical resistance in comparison with a bandpass filter utilizing a cholesteric liquid crystal, thus exhibiting high practical value.
However, a liquid crystal display having a bandpass filter, which is made of lamination of vapor-deposited thin films or thin films of resin compositions and arranged on the emitting side of the backlight, poses a problem that incident light from the display side of the liquid crystal display (that is, the opposite side to the side on which the backlight is arranged) reflects on the surface of the bandpass filter and is visually observed as return light, thus deteriorating the display quality of the liquid crystal display.
More specifically, as illustrated in FIG. 9, when in white display, external light L1 introduced from the display side of a liquid crystal display reaches a bandpass filter 1 through a polarizer 8, a liquid crystal cell 4 and a polarizer 3, reflects on the surface of the bandpass filter 1 and is visually recognized as return light L2. Accordingly, where an image of the periphery of the liquid crystal display is mirror-likely reflected thereon, or an anti-glare layer is provided on the surface of the polarizer 8, the mirror-likely reflected image is spread over a large area on the anti-glare layer, or reflected colors of the bandpass filter 1 are visually recognized. These phenomena pose a problem to substantially deteriorate the display quality of the liquid crystal display.